Image rejection circuit



Nov. 29, 1966 B. BIRKENES 3,289,087

IMAGE REJECTION CIRCUIT Original Filed March 24, 1959 SUPERHETROOYNE ByBernhard B/r/renes United States Patent 3,289,087 IMAGE REJECTIONCIRCUIT Bernhard Birkenes, Chicago, Ill., assignor to Motorola, Inc.,Chicago, 11]., a corporation of Illinois Original application Mar. 24,1959, Ser. No. 801,553, new

Patent No. 3,089,087, dated May 7, 1963. Divided and this applicationFeb. 7, 1963, Ser. No. 266,121

2 Claims. (Cl. 325-388) This is a division of my co-pending application,Ser. No. 801,553, filed Mar. 24, 1959, now Patent No. 3,089, 087.

This invention relates to an image rejection network for a radioreceiver.

It is an object hereof to reduce the spurious signal response of atransistor radio receiver.

A feature of the invention is a provision of a tuned antenna inputcircuit and image rejection trap through which radio frequency signalsmay be applied to the first stage of a transistor radio receiver.

In a particular form of the invention radio frequency signals arecoupled from the antenna through an input network to a stage ofamplification. The input network comprises a pair of resonant circuits,one being tuned to the desired frequency and the other being tuned tothe image frequency of the superhetrodyne circuit of the radio receiver.A first series connected inductor-capacitor combination is coupled inparallel with an input capacitor which input capacitor is coupled to theantenna, thereby forming a tuned circuit for the desired frequency. Asecond series connected inductor-capacitor combination, with the inputcapacitor coupled in parallel therewith, provide the image frequencyrejection circuit. The two inductors are closely coupled and poled sothat any image signals present are cancelled. One of the inductors iscoupled to the amplifier device of the stage.

A more complete understanding of the details may be had by reference tothe drawing which is a schematic diagram of a part of a superhetrodyneradio receiver incorporating the invention.

In the drawing the receiver shown is one adapted for use in anautomobile and for operation directly from a normal 12 volt battery andgenerator. The receiver includes a radio frequency amplifier stagehaving a transistor 12. A tuned network for selecting a desired signalis connected between the base of transistor 12 and the antenna 14. Thisnetwork includes a slug tunable inductor 16 connected to the antenna 14and capacitors 17 and 18 series connected between the inductor 16 andground. Inductor is coupled between the junction of capacitors 17, 18and the capacitor 21 which is connected to ground. Inductor 23 isclosely coupled to inductor 20 and is connected between the base oftransistor 12 and ground through the bypass capacitor 25. Inductors 20,23 are also slug tuned. Capacitors 17 and 21 are variable for alignment.

This input network forms a double tuned circuit with inductor 16 andcapacitors 17 and 18 parallel tuned to the input signal. Capacitor 18also provides coupling to a second tuned circuit comprising capacitor18, inductor 20 and capacitor 21 which are also tuned to the signal. Thesignal is then inductively coupled to winding 23 and applied to thetransistor 12. This network thus provides a desirable impedance match tothe antenna 14 by means of a parallel resonant circuit and furtherprovides a desirable low impedance match for the input electrode oftransistor 12.

The input network further forms an image rejection trap. The imagefrequency in a superheterodyne 3289,87 Patented Nov. 29, 1966 receiverof the type being described will be spaced from the frequency to whichthe local oscillator is tuned by an amount equal to the intermediatefrequency of the receiver. For example, if the local oscillator is tunedabove the desired signal, the image would be at a frequency equal totwice the intermediate frequency plus the desired signal frequency.Inductor 23 together with capacitor 18 and capacitor 27, which lattercapacitor is connected between the base electrode of transistor 12 andjunction of capacitors 17, 18, are tuned to the image frequency. Thus,there is a tendency for the image signal to be developed across theinductor 23. However, the image signal is also developed across inductor20 and this is coupled into inductor 23 with an opposing phase so as tominimize the image signal which tends to be developed in inductor 23.

It should also be noted that tuning of the inductors 20, 23 will effecttuning of the image rejection trap so that the input network to thereceiver provides double tuning of the desired signal, rejection of theimage frequency, and impedance matching of the antenna to the transistor12.

Amplifier stage 10 also includes an emitter stabilizing resistor 30connected between the emitter electrode and potential supply lead 32.The emitter electrode is bypassed for signals by means of capacitor 34.The output of'the stage 10 is derived in a transformer 36 having aprimary winding which is connected between ground and the collector oftransistor 12. This winding is shunted by means of a resistor 38. Aneutralizing capacitor 39 is also connected between the collector andbase electrodes of the transistor.

In a receiver of practical construction operating in accordance with theteachings of the invention, component parts were as follows:

Transistor 12 2N247. Capacitor 18 mmfd. Capacitor 25 .1 mfd. Capacitor27 .002 mfd.

The above described receive-r comprises an improved circuit which issuitable for use in portable or mobile applications. The desiredcircuitry for reducing spurious response is especially effective forsuch uses.

I claim:

1. A wave signal receiver of the superheterodyne type having an antennainput network coupled to a stage for translating a received signal,which stage includes a transistor having a base element, said networkbeing adapted to be double tuned to a received signal and comprising afirst inductor series coupled with first capacitor means and a secondinductor series coupled with a second capacitor across said firstcapacitor means, said network further having a network series tunable tothe image of the received signal including a third capacitor and a thirdinductor series coupled across said first capacitor means, said secondand third inductors being tightly coupled inductively to cancel imagesignals in said third inductor and said third inductor providing animpedance match for said base element, means for simultaneouslyadjusting the tuning of said first, second and third inductors, meansdirect current coupling said base element to said third inductor, andmeans for applying a bias potential for said base element to said thirdinductor.

2. A wave signal receiver of a superhetrodyne type having an antennainput network coupled in a stage for translating a received signal, saidstage including an amplifier transistor, said network comprising a firstinductor series coupled with first capacitor means, a second inductorseries coupled with a second capacitor across at least a portion of saidfirst capacitor means, said network having a further circuit tunable tothe image of the received signal including a third capacitor and a thirdinductor series coupled across a portion of said first capacitor means,said second and third inductors being inductively coupled to cancelimage signals therebetween, means connecting said transistor to saidthird inductor to apply signals to said transistor, and means foradjusting the tuning of said first, second and third inductors.

No references cited.

5 KATHLEEN H. CLAFFY, Primary Examiner.

R. S. BELL, Assistant Examiner.

2. A WAVE SIGNAL RECEIVER OF A SUPERHETERODYNE TYPE HAVING AN ANTENNAINPUT NETWORK COUPLED IN A STAGE FOR TRANSLATING A RECEIVED SIGNAL, SAIDSTAGE INCLUDING AN AMPLIFIER TRANSISTOR, SAID NETWORK COMPRISING A FIRSTINDUCTOR SERIES COUPLED WITH FIRST CAPACITOR MEANS, A SECOND INDUCTORSERIES COUPLED WITH A SECOND CAPACITOR ACROSS AT LEAST A PORTION OF SAIDFIRST CAPACITOR MEANS, SAID NETWORK HAVING A FURTHER CIRCUIT TUNABLE TOTHE IMAGE OF THE RECEIVED SIGNAL INCLUDING A THIRD CAPACITOR AND A THIRDINDUCTOR SERIES COUPLED ACROSS A PORTION OF SAID FIRST CAPACITOR MEANS,SAID SECOND AND THIRD INDUCTORS BEING INDUCTIVELY COUPLED TO CANCELIMAGE SIGNALS THEREBETWEEN, MEANS CONNECTING SAID TRANSISTOR TO SAIDTHIRD INDUCTOR TO APPLY SIGNALS TO SAID TRANSISTOR, AND MEANS FORADJUSTING THE TUNING OF SAID FIRST, SECOND AND THIRD INDUCTORS.